I just had an epiphany: Blizzard is building a Raid Finder for a mid-Cataclysm patch.
They just announced that raid IDs will be more forgiving: if your raid ID has killed the same set of bosses as another raid ID, you can switch over to the other raid ID.
Now add a Raid Finder that matches you with another group of players that all have a compatible raid ID. You join a raid, do your best, kill a few bosses, and then tomorrow, you log on again and join a different group that has the same set of bosses downed. It doesn't matter whether it's the same group as yesterday, as long as they've made the same amount of progress.
And there you have it: no more distinction between transient and extended content. Why should there be? Raiding is not rocket science, and the biggest barrier is that it's hard to find a regular group of people who are all online at the same time as you. Right now, nobody wants to PUG, because you'll just get locked to a dungeon with one boss down, and never see that group again. But with this feature, you can get a new group every day, and you can raid whenever you have the time for it. Brilliant!
(And Blizzard, if you're not working on this feature, you should be. It will actually make raiding feasible for all the players who are interested in and capable of raiding, but can't carve out regular weekly times for a guild raid.)
A system that implemented this isn't that hard to conceptualize (at least theoretically, real code always makes easy ideas hard). Right now, raid instances have an existence in the world. My character is tied to this specific raid instance, which exists even when my character is not online. And this has consequences. In the worst case, the raid id can be "stolen", and the remainder of the raid completed by another group, leaving most of the people in first group out in the cold.
But suppose a player's raid id was simple bit field, with 1 representing a live boss, and 0 representing a dead boss. So a player's TotC1 id could be 00011, saying that they've killed the first three bosses, but Twin Valks and Anub are still alive.
For any given group of players, it becomes trivial to find the common set of bosses still alive. Simply bitwise-AND the raid ids together, and you can use the resulting mask to populate an instance with the bosses that no one has killed.
For example, Player 1 kills Beasts and Faction Champs (raid id 01011), and player 2 has killed Jaraxxus, Faction Champs, and Twin Valks (raid id 10001). 01011 ∧ 10001 = 00001, giving you an instance with only Anub'arak active. You can just keep bit-wise ANDing with all the players in the raid.
You can put in checks if you don't want early bosses to be alive and later bosses dead. Or you might not bother. After all, does it really matter if someone with Lich King dead gets to knock off Marrowgar the day after?
The key here is that there is no logical reason the specific raid instance needs to persist as its own entity. It can be generated at raid time so long as you know the set of bosses to populate the instance with. And since the bit fields are connected to the experiences of the individual players that week, you can easily generate the correct set of bosses for any given group of people.
You can also use the mask to determine whether to deny a new addition to the raid entry into the instance. If the current raid mask AND the new player's raid id is not equal to the mask, she cannot enter.
Even a Raid Finder wouldn't be that complicated to implement. The number of 1's in the result mask would make a strong foundation for the fitness function of the grouping algorithm. You want to group people such that they get an instance with as many bosses active as possible.
The hardest part, in my mind, would be making sure that an individual's raid id gets properly updated in each fight. You'd have to account for people intentionally disconnecting and joining at points in the fight in order to evade being tagged with the kill.
But as Jeremy points out, such a system would change raiding from being de jure extended content into transient content. It might still be de facto extended content, simply because a consistent group would be the best path to success. But it would put a lot less blocks in the way of individuals wanting to raid in a transient fashion, while still preserving the rule that "you can kill a boss a maximum of once per week".
1. I'm using TotC because I don't want to write out 12 digit bit fields.